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SubjectRe: [PATCH] ftrace: Fix possible warning on checking all pages used in ftrace_process_locs()
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On 2023/7/10 22:46, Steven Rostedt wrote:
> On Tue, 11 Jul 2023 05:29:58 +0800
> Zheng Yejian <zhengyejian1@huawei.com> wrote:
>
>> As comments in ftrace_process_locs(), there may be NULL pointers in
>> mcount_loc section:
>> > Some architecture linkers will pad between
>> > the different mcount_loc sections of different
>> > object files to satisfy alignments.
>> > Skip any NULL pointers.
>>
>> After 20e5227e9f55 ("ftrace: allow NULL pointers in mcount_loc"),
>> NULL pointers will be accounted when allocating ftrace pages but
>> skipped before adding into ftrace pages, this may result in some
>> pages not being used. Then after 706c81f87f84 ("ftrace: Remove extra
>> helper functions"), warning may occur at:
>> WARN_ON(pg->next);
>>
>> So we may need to skip NULL pointers before allocating ftrace pages.
>>
>> Fixes: 706c81f87f84 ("ftrace: Remove extra helper functions")
>> Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com>
>> ---
>> kernel/trace/ftrace.c | 10 ++++++++++
>> 1 file changed, 10 insertions(+)
>>
>> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
>> index 3740aca79fe7..5b474165df31 100644
>> --- a/kernel/trace/ftrace.c
>> +++ b/kernel/trace/ftrace.c
>> @@ -6485,6 +6485,16 @@ static int ftrace_process_locs(struct module *mod,
>> if (!count)
>> return 0;
>>
>> + p = start;
>> + while (p < end) {
>> + /*
>> + * Refer to conments below, there may be NULL pointers,
>> + * skip them before allocating pages
>> + */
>> + addr = ftrace_call_adjust(*p++);
>> + if (!addr)
>> + count--;
>> + }
>
> My main concern about this is the added overhead during boot to process
> this. There's 10s of thousands of functions, so this loop will be 10s of
> thousands. I also don't like that this is an unconditional loop (meaning it
> executes even when it is unnecessary to do so).
>

Agreed! The added overhead probably superfluousin in most cases.

>
>> /*
>> * Sorting mcount in vmlinux at build time depend on
>> * CONFIG_BUILDTIME_MCOUNT_SORT, while mcount loc in
>
> How about something like this?
>
> -- Steve
>
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index b24c573934af..acd033371721 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -6474,6 +6474,7 @@ static int ftrace_process_locs(struct module *mod,
> struct ftrace_page *start_pg;
> struct ftrace_page *pg;
> struct dyn_ftrace *rec;
> + unsigned long skipped = 0;
> unsigned long count;
> unsigned long *p;
> unsigned long addr;
> @@ -6536,8 +6537,10 @@ static int ftrace_process_locs(struct module *mod,
> * object files to satisfy alignments.
> * Skip any NULL pointers.
> */
> - if (!addr)
> + if (!addr) {
> + skipped++;
> continue;
> + }
>
> end_offset = (pg->index+1) * sizeof(pg->records[0]);
> if (end_offset > PAGE_SIZE << pg->order) {
> @@ -6551,12 +6554,24 @@ static int ftrace_process_locs(struct module *mod,
> rec->ip = addr;
> }
>
> - /* We should have used all pages */
> - WARN_ON(pg->next);
> -
> /* Assign the last page to ftrace_pages */
> ftrace_pages = pg;
>
> + /* We should have used all pages unless we skipped some */
> + if (pg->next) {
> + WARN_ON(!skipped);
> + while (ftrace_pages->next) {
> + pg = ftrace_pages->next;
> + ftrace_pages->next = pg->next;
> + if (pg->records) {
> + free_pages((unsigned long)pg->records, pg->order);
> + ftrace_number_of_pages -= 1 << pg->order;
> + }
> + kfree(pg);
> + ftrace_number_of_groups--;
> + }

Do we only need to free the pages that not being used?

> + }
> +
> /*
> * We only need to disable interrupts on start up
> * because we are modifying code that an interrupt
>

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